1. Field of the Invention
The present invention relates to a belt drive controller for controlling the driving of a belt so as to intermittently move an endless belt wrapped around a plurality of supporting rotating bodies, and an image forming apparatus of an ink jet recording type and so forth provided therewith.
2. Description of the Background Art
A known example of this type of image forming apparatus is an ink jet recording type of image forming apparatus which forms an image on a recording material by intermittently moving a recording material transport member to repeatedly advance the recording material in steps when forming an image on a recording material supported and transported on a recording material transport member comprising an endless belt.
In this type of image forming apparatus, a conveyor belt (recording material transport member) may be used as a recording material transport mechanism. In this type of recording material transport mechanism, a code wheel (encoder disk) is installed on supporting rollers (supporting rotating bodies) on which the conveyor belt is wrapped around, and a code on the code wheel is read with an encoder. The conveyor belt is moved intermittently based on the output of the encoder, and driving is controlled so that the recording material stops at each target stopping position. As a result of carrying out this type of driving control, since the accuracy of each stopping position during intermittent transport of the recording material can be improved, the accuracy of the impact position of the ink droplets on the recording material can be improved, there by making it possible to enhance image quality. An image forming apparatus employing this type of driving control method is described in, for example, Japanese Patent Application Laid-open No. 2001-248822. The image forming apparatus described in this publication transports a recording material in a secondary scanning direction by arranging a transport roller and a discharge roller on the upstream side and down stream side of a platen. A code wheel is installed on the transport roller shaft, a code on the code wheel is read with an encoder, and the transport roller and conveyor belt are moved intermittently based on the encoder output.
In addition, Japanese Patent Application Laid-open No. 2005-115398 describes a method for controlling driving of an endless belt provided in a so-called electrophotographic type of image forming apparatus. This driving control method controls driving by detecting the rotation angular displacement or rotation angular velocity of a driven supporting rotating body among a plurality of supporting rotating bodies around which a belt is wrapped, and suppresses fluctuations in the movement velocity of the belt caused by periodical fluctuations in thickness in the circumferential direction of the belt based on the detection result so that the belt moves at a constant movement velocity.
Ink jet recording types of image recording apparatuses have recently come to use pigment-based ink instead of dye-based ink so as to improve ink light resistance and deterioration over time, resulting in a trend in which ink viscosity is increasing. Although bleeding on to the recording material has decreased considerably as a result of increasing the viscosity of the ink, poor accuracy resulting from shifts in the impact position of the ink droplets on the recording material is readily apparent visually in the form white lines, black lines and banding. This deterioration in image quality has a particularly large effect on the accuracy of each belt stopping position during intermittent movement by the conveyor belt which supports and transports the recording material in the direction of secondary scanning (direction of transport of the recording material) in particular. Consequently, controlling each belt stopping position during intermittent movement of the conveyor belt to attain even higher accuracy has become an important technical issue. However, conventional methods for controlling driving as described in the above-mentioned Japanese Patent Application Laid-open No. 2001-248822 had the problem of being unable to control each belt stopping position during intermittent movement of the conveyor belt with sufficiently high accuracy so as to be able to adequately suppress deterioration of image quality in the form of white lines, black lines and banding.
A first cause of being unable to control each belt stopping position with sufficiently high accuracy during intermittent movement of the conveyor belt is fluctuation during belt driving in the distance from the roller surface to the belt pitch line, namely the pitch line distance (PLD), at the portion of the belt wound around the drive roller (driving supporting rotating body). More specifically, the movement velocity of a belt is typically determined by the PLD. This PLD is equivalent to the distance between the center of the belt in the direction of thickness and the belt inner surface, namely the roller surface, in the case of a single-layer belt made of a uniform material and the absolute value of belt flexibility of the inside and outside of the belt being nearly equal. Thus, since the relationship between PLD and belt thickness is nearly constant in the case of such a single-layer belt, the movement velocity of the belt can be determined according to the amount of unevenness in belt thickness. However, as a result of different mutual flexibility between hard layers and soft layers in a belt composed of a plurality of layers, the distance from a position shifted from the center of the belt in the direction of thickness and the roller surface becomes the PLD.
If PLD fluctuates in the portion of the belt wound around the drive roller during belt driving, the belt movement velocity increases when a portion of the belt having a large PLD is wound around the drive roller, while conversely the belt movement velocity decreases when a portion of the belt having a small PLD is wound around the drive roller. As a result, the belt movement distance becomes longer when a portion of the belt having a large PLD is wound around the drive roller, while conversely the belt movement distance becomes shorter when a portion of the belt having a small PLD is wound around the drive roller. Consequently, if fluctuations in the PLD occur in a portion of the belt wound around the drive roller during belt driving, even if the drive roller rotates by the same rotation angle, the distance over which the belt moves as a result of that rotation changes. As a result, even if the drive roller is rotated by the same rotation angle during each movement of the conveyor belt during intermittent transport of the recording material, each distance by which the recording material is transported ends up being different. For this reason, the belt stopping position during intermittent movement of the conveyor belt ends up shifting from the target position, and each belt stopping position cannot be controlled accurately during intermittent movement of the conveyor belt.
In addition, a second cause of being unable to accurately control each belt stopping position during intermittent movement of the conveyor belt is detection error attributable to the encoder or other detection means. In the case of detecting a single rotation angular displacement or rotation angular velocity of a supporting roller around which a conveyor belt is wrapped, and controlling driving based on that detection result, an error occurs between the belt movement distance obtained from the detection result and the actual belt movement distance due to eccentricity of the supporting roller and the assembly accuracy of the detection means with respect to that supporting roller. If driving is controlled based on a detection result which contains this type of error, the belt stopping position during intermittent movement of the conveyor belt ends up shifting from the target position, thereby preventing accurate control of each belt stopping position during intermittent movement of the conveyor belt.
In addition, a third cause of being unable to accurately control each belt stopping position during intermittent movement of the conveyor belt is a change in the diameter of a supporting roller attributable to temperature changes, wear over time and so on. If the diameter of a supporting roller changes, even if the supporting roller rotates by the same rotation angle, the belt movement distance at that time differs. As a result, if the diameter of the drive roller changes, for example, even if that drive roller is rotated by the same rotation angle, the distance the belt is driven due to that rotation changes. As a result, even if the drive roller is rotated by the same rotation angle during each movement of the conveyor belt during intermittent transport of the recording material, the distance the recording material is transported ends up differing each time, thereby preventing accurate control of each belt stopping position during intermittent movement of the conveyor belt. In addition, if the diameter of a driven roller provided with detection means changes, a result is obtained which is different from the distance the belt has actually moved even in the case of a detection result for the same rotation angle, thereby resulting in detection error. Accordingly, if driving is controlled based on a detection result which contains this type of error, the belt stopping positions during intermittent movement of the conveyor belt end up shifting from the target position, and each belt stopping position during intermittent movement of the conveyor belt cannot be accurately controlled.
Furthermore, the above-mentioned problems of the prior art are not limited to driving control which intermittently moves a belt used as a recording material transport member which supports and transports a recording material, but also can similarly occur for all types of driving control of a belt which is moved intermittently.
Technologies relating to the present invention are also disclosed in, for example, Japanese Patent No. 3,564,953, Japanese Patent No. 3,658,262, Japanese Patent Application Laid-open No. H08-282009 and Japanese Patent Application Laid-open No. 2000-330353.